Detuning interfering magnetron modes



Dec. 18, 1962 R. c. SIBLEY 3,069,595

my v a a? b1) I 7 gj ll Illa @[NVENTOR ROBERT C'. .SI'BLEY ATTORNEY United rates 3 969,595 DETUNKNG 9 llNG hC'A-GNETRtlh-l Mtllhllhl Robert C. Sihle Wiliramsport, Pm, assignor to Sylvania Electric Products line, a corporation of Eelaware Filed Ann, 22, 19-35 3, Ser. No. 503% 6 tllanns. (Cl. 3i5--=-".l .d9)

mode, where N is the number of resonators in the anode, and the 7r-l mode is known as the mode. More specifically, in a sixteen resonator magnetron the 1r mode is known as the 8 mode and the n'l is mode 7. Strapping is normally used to provide a sufficient frequency separation between the 1r and the 7r-1 mode so that the starting voltage for the 7r-1 mode is higher than it is for the 11' mode. However, since all magnetrons require a finite time to start, this time desirably being as short as possible, there is a tendency for the voltage from the modulator to overshoot the value necessary for normal 1.- mode operation until the magnetron can surllciently load the modualtor to reduce the voltage to the proper value. in the case of the heavily strapped anode, when the voltage thus does overshoot it often goes high enough to reach the startin voltage for the 1.l mode. When the starting voltage for the rr-l mode is reached there is a tendency for this mode to build up. The 1r-1 mode consists of doublet degenerate components, one of which is coupled to the output of the magnetron and the other of which is uncoupled.

A number of means have been suggested for reducing the opportunity of this mode to build up in power during the starting of the magnetron, one of them being to shift the phase of the two degenerate modes such that both of them are coupled to the output. The usual approach for doing this is to alter the configuration of the resonant cavities at positions around tie anode bloclr which are unsymmetrical with respect to the output coupler. Whilethe extra loading caused by thus shifting the phase of the degenerate modes does decrease the possibility for moding, both of these modes still remain in competition with and under certain conditions are somewhat favored over the 1r mode. This is true because of the characteristic of the magnetron that the starting voltage for a particular mode is inversely proportional to the wavelength at that mode times the Hartree number associated with one of the harmonics of the mode. Considering again, by way of example, a sixteen resonator magnetron, mode 7 is the usual interfering mode to the 1r mode of 8, the most important Patented l8, 1%82 Hartree harmonic for the 1r mode is harmonic number 8, and for the 7 mode, the most important l-Iartree harmonic is number 9. Since the wavelength for the 7 mode (7r-1) is lower than the wavelength for the 8 mode (11' mode), the product of for the 7 mode and the l-lartree number 9 is quite close to the product of the wavelength for the 8 mode and the lrlartree number 8 for the 8 mode, Whereby their starting voltages are relatively close to each other. in the case of heavily strapped anodes, the AH product for the 7 mode is lower than the All product for the 8 rode with the consequence that the overshoot of voltage during build up is more favorable for the undesired mode than it is for the desired mode, at least for a brief period. This is particularly true in applications where the modulator voltage pulses have a very fast rise time; that is, there is a strong tendency to overshoot the desired voltage for the ir mode before the modulator becomes sufficiently loaded to bring the voltage down to the desired level.

it is an object of this invention to minimize the opportunity for build up of power in the interfering mode caused by overshooting the voltage necessary for operation in the desired mode.

it is a further and general object of this invention to provide an improved magnetron.

These and other objects of this invention are attained in specific embodiments of the invention by removing a portion of the straps in the vicinity of the highest electric fields of the uncoupled doublet of the predominant interfering mode to tune the interfering mode to a new frequency to provide a smaller mode separation between the 1r and tr1 modes.

In one specific embodiment, sections are removed from both tae inner and outer straps, at both ends of the anode block, at positions away from the output resonator of the magnetron. This removal of sections of the strap at these positions where the electric fields of the doublet components of the undesired mode are maximum results in an adjustment of the c'r-l mode frequency relative to the TI mode frequency. With this appreciably lower frequency for the 1r-l mode, the product of RH for the 1.--l mode is higher than the corresponding product for the 1r mode. Since a finite time is required for starting, the rapidly rising voltage rapidly advances past the starting voltage for the :r-1 mode without reaction, and overshoots the starting requirement for the desired mode, and then returns again to the starting voltage for the 7r mode at which time the magnetron will have had adequate time to start operating and to load the modulator down to the desired voltage. Thus, the tendency to start in the wrong mode is reduced substantially due to the inherent. tarting time of magnetrons and the fact that the modulating pulses in most systems increase very rapidly in potential.

iihese and various other features of th' invention may he understood from consideration of the following detailed description and the accompanying drawing, in which:

FIG. 1 is a plan view of a magnetron anode block incorporating the invention.

FIG. 2 is a sectional view along line 2-2 of FIG. 1.

Turning now to the drawing, the invention is shown embodied in a vane type magnetron having a circular anode it) having an axial aperture ll, a plurality of semi-circular cavity resonator bores through 44-, and a plurality of vanes 46 to 76 extending radially inward from the back wall of the anode, equally spaced in a circuit array around the aperture 1'2 and communicating therewith. A cathode 78 comprising a cylindrical sleeve having an electron emissive coating thereon is positioned within the axial aperture Energy is coupled from the magnetron through an output coupler extending through the anode wall and communicating with the resonator bore 44.

As best seen in FIG. 1, the anode is strapped in the usual manner by a pair of concentric conducting rings 82 and 34, the inner ring being secured near the tip of alternate or odd-numbered ones of the vanes and passing over cut outs 85 in the intermediate or even-numbered vanes, and the outer ring 84 passing over the even-numbered vanes to which the inner ring is secured and joined to the odd-numbered vanes. Similar strapping rings are also afiixed to the other end of the anode, in this case, however, the straps being secured to the vanes which the corresponding rings pass over at the other end, as shown in FIG. 2.

In a heavily strapped magnetron of the type just described, one of the degenerate modes of the wr-l mode (mode No. 7 in the illustrated sixteen resonator case) has high voltage positions which are ninety geometrical degrees away from the discontinuity caused by the output coupler 86}. In accordance with this invention, both straps are modified at this position to effect a change in frequency of the degenerate mode. In the illustrated specific embodiment, a short section of the outer ring over the vane 79 is removed, and similarly a short section is removed from the inner ring 82 over vane 68. It is to be noted tthat the removed sections are above vanes to which the straps normally are not attached. As viewed in FIG. 2, the sections of the straps are removed in a region substantially ninety geometrical degrees counterclockwise from the output coupler 8- .3. Similar sections are removed from the straps at the other end of the anode block at a position also substantially 96 from the output coupler in a counterclockwise direction as viewed from the other side; namely, over vanes 52 and 54. Thus, the removed portions at one end of the block are diametrically opposite from the portions of the straps removed at the other end of the block. The removal of these four short sections of the straps of the magnetron results in turning the 1r1 mode to an appreciably different frequency than exists when the straps are unmodified, this detuning, however, having only a minor effect on the fundamental mode frequency.

In one specific embodiment of the invention it was found that appropriate mode separation was achieved by removing a short section of both straps over one unsecured vane, at each end of the magnetron, as abovedescribed, but should an application require an even greater decrease in the frequency of the undesired mode, longer sections of the strap may be removed inasmuch as the high electric field region of the anode for the rr-l mode extends o'ver an appreciable sector of the semicircular portions of the magnetron extending in both directions from the output coupler. Thus, the strapping rings may be removed only over one Vane as shown, over one Whole cavity, or over more than one cavity as necessary to obtain the desired frequency for the unwanted mode.

Likewise, since the doublet components of the unwanted mode are present at either side of the output coupler, additional tuning can be obtained by removing similar sections from the strapping rings at points diametrically opposite from those shown in the drawing, at both ends of the block.

In a tunable magnetron having a tuning range from 8500 to 9600 mo. and wherein the strapping rings have a diameter of approximately one-half inch, the removal of four short sections of the rings as illustrated provided a reduction in mode separation from approximately 16% in the heavily strapped case to approximately 7%. With sixteen resonators, 12.5% mode separation results in equal starting voltages for the 7r and 1r-1 modes. Values of mode separation higher than 12.5% result in the usual condition for heavily strapped anodes, that of higher starting voltage for the 7r-1 mode. The resulting increase in the Hi product for the 1rl mode substantially eliminates all tendency for the magnetron to operate in the 1rl mode.

It is to be understood that the above-described embodiment of the invention is illustrative of the application of the principle of the invention. Numerous other arrangements may be devised by those skilled in the art without departing from the spirit and scope of the invention.

What is claimed is:

1. in combination, an annular magnetron anode comprising a plurality of vanes defining a plurality of cavity resonators, an output gap communicating with one of said cavity resonators for coupling energy therefrom, a first strap of electrically conducting material connected to one end of all odd-numbered vanes of said anode near the tips thereof and extending across the even-numbered vanes, a second strap of electrically conductive material connected to said one end of all even-numbered vanes of said anode and extending across the odd-numbered vanes, said first and second straps each having a section thereof removed at a region displaced substantially ninety geometrical degrees around said anode from said output gap.

2. A strapping arrangement for a magnetron for detuning interfering modes, said magnetron having an annular anode and a plurality of vanes defining a plurality of cavity resonators and including an output gap communicating with one of said cavity resonators for coupling energy therefrom, a first conductive strap connected to one end of all odd-numbered vanes of said anode near the tips thereof and extending with clearance across the even-numbered vanes, a second conductive strap of larger diameter than said first strap connected to said one end of all even-numbered vanes of said anode and extending with clearance across the odd-numbered vanes, said first and second straps each having a section thereof removed at a region displaced from said output gap by a geometrical angle of approximately the removed section in each strap including at least a portion extending across a vane to which it is not secured.

3. A strapping arrangement for a magnetron for detuning interfering modes, said magnetron having an annular anode and a plurality of vanes defining a plurality of cavity resonators and including an output gap communicating with one of said cavity resonators for coupling energy therefrom, a first conductive strap connected to one end of all odd-numbered vanes of said anode near the tips thereof and extending With clearance across the evennumbered vanes, a second conductive strap of larger diameter than said first strap and concentric therewith connected to said one end of all even-numbered vanes of said anode and extending with clearance across the oddnumbered vanes, said first and second straps each having a section thereof removed at a region where it respectively extends across vanes of said anode which are displaced from said output gap by a geometrical angle of substantially 90.

4. Apparatus in accordance with claim 3 wherein said removed sections have a length slightly greater than the thickness of said vanes.

5. A strapping arrangement for a magnetron for detuning interfering modes, said magnetron having an annular anode and a plurality of vanes extending radially inward defining a plurality of cavity resonators and including an output gap communicating with one of said cavity resonators for coupling energy therefrom, a first conductive strap connected to all odd-numbered vanes of said anode near the tips thereof at one end of said anode and extending with clearance across the even-numbered vanes, a second conductive strap of larger diameter than said first strap connected to all even-numbered vanes of said anode at said one end of said anode and extend mg with clearance across the even-numbered vanes, 21 third conductive strap having a diameter substantially equal to that of said first strap connected to all even-numbered vanes of said anode near the tips thcrecf at the ether end of said anode, and extending with cleaance across the odd-numbered vanes, and a fourth conductive strap having a diameter substantially equal to that of said second strap connected to all odd-numbered vanes of said anode at said other end and extending with clearance across the even-numbered vanes, each of said straps having a section thereof removed Where it respectively ex- 5 removed sections have a length slightly greater than the thickness of said vanes.

References Cited in the file of this patent UNITED STATES PATENTS 2,447,537 Ronci Aug. 24, 1948 2,467,538 Shoupp Apr. 19, 1949 2,550,614 Spencer Apr. 24, 1951 

